If you are always well past the threshold of (re)qualifying for whatever elite status you are (and not within a vacation flight's reach of qualifying for the next level), then that may not be a problem.

But for those of us who most years JUST BARELY make it to whatever elite level we're shooting for, using miles for one of those flights outright (as opposed to upgrades) would keep us from (re)qualifying. (It doesn't matter WHOSE miles we would use, because I know of no airline which lets you earn miles on outright mileage awards, so it's the same danger of not requalifying whether you burn up miles on that same airline, or on some other airline you only earned miles through non-flying.)

For example, in my case I'm primarily with AA, where I just barely get to PLAT (usually all or almost all personal trips!) each year. If I took just one of those many-mile-earning overseas trips a year as an award, I wouldn't get there. At AA, I do burn miles on int'l upgrades, but I can't burn them as fast as I earn them.

Meanwhile, I'm building up miles (through non-flying) in NW and UA and BA for perhaps going to destinations like Australia and Thailand and South Africa someday when I have the time (those are destinations I can't pay-and-upgrade over at AA anyhow, and there's a chance of doing that at NW or UA or BA). I have NO GOOD USE for those miles on anything I can quickly use them for (since I never use miles in place of cheap economy flights, only for high-value redemptions). If I burn all my UA miles now, I have no way to go to Australia via an upgrade next year (if that turns out to be a year I can "afford" to, in time and no problem requalifying at AA). Same for each other airline I have collected miles at.